This invention relates to refrigeration systems employing multiple compressors in parallel and particularly to a system for achieving the proper level of lubricant in each of the compressors of such a system when all of the compressors are operating and also when one or more of the compressors are out of operation or operating at partial capacity.
During operation of refrigeration compressor systems, it is common for compressor lubricant to be carried from the compressors by the compressed refrigerant, so that, unless it is separated at some point, the lubricant circulates with the refrigerant throughout the system. Where a refrigeration system employs multiple compressors in parallel, it is important that the appropriate amount of lubricant be returned to each of the compressors in order that each compressor has at all times a supply sufficient for its lubrication needs, but not so much lubricant as to impair its operation.
There is a recognized tendency in multiple parallel refrigeration compressor systems for lubricant to excessively accumulate within one or more of the compressors of the system at the expense of the other compressors in the system. In an effort to overcome this tendency, a variety of systems have been employed to maintain the correct level of lubricant in all of the compressors of a parallel system. Such systems are particularly important where it is intended that a parallel compressor system will be operated with less than all of its compressors or with some compressors at less than full capacity, for it is common to have lubricant accumulate in non-operating and partial capacity compressors.
One solution to the afore-referenced problem of lubricant accumulating in certain compressors of a parallel compressor system has been to mount a lubricant separator on the discharge side of the compressors to separate the lubricant from the discharge gas and return it to the compressor. The use of lubricant separators on the compressor discharge has, however, been accompanied by undesirable side effects, such as increased crankcase temperature caused by the elevated temperature of the lubricant in the compressor discharge gas. Further, as there is a tendency for parallel compressors to operate at slightly different pressures, there is a concommitant tendency for lubricant from the separator to be directed preferentially to the compressor having the lowest pressure which, of course, results in low lubricant levels developing in the other compressors of the system. To compensate for these deficiencies, complex and expensive controls have often been provided in such systems; aside from their cost, such controls are considered to be undesirable as they are prone to failure.
Other attempts to solve the problem of uneven lubricant distribution in parallel compressor systems are described in U.S. Pat. Nos. 4,179,248 and 4,411,141. These systems also utilize intricate valves and controls to regulate the level of lubricant in the compressor and are expensive and prone to failure.
Accordingly, it is an object of the present invention to provide an improved lubricant return system for parallel refrigeration compressors.
Another object of the present invention is to provide an improved lubricant return system which avoids complicated and intricate controls to regulate the level of lubricant in parallel refrigeration compressors.
Another object of the present invention is to provide an improved lubricant return system for parallel refrigeration compressors which avoids excessive buildup of lubricant in compressors of the system which are not operating or which are operating at partial capacity.